Conventionally, a TeO2 crystal or a PbMoO4 crystal has been used as an acoustooptic medium for visible light emitted from an argon laser or a helium-neon laser. On the other hand, it has been studied to combine an acoustooptic device with a light source that utilizes light in the blue to ultraviolet region, e.g., a YAG laser that emits third to fourth harmonics or a semiconductor laser that emits light with various short wavelengths. Examples of the acoustooptic device include acoustooptic modulators, acoustooptic deflectors, acoustooptic filters, and acoustooptic frequency shifters.
As an acoustooptic medium of a conventional acoustooptic modulation device that utilizes light within the ultraviolet region, quartz glass, a quartz crystal, a KDP crystal, or the like has been used (see Non-Patent Documents 1 and 2, for example). In recent years, the use of various types of borate-based crystal for ultraviolet acoustooptic devices has been studied (see Patent Document 1, for example).
However, in an acoustooptic device in which a PbMoO4 crystal is used, since the absorption edge wavelength is around 410 nm in the PbMoO4 crystal, there has been a problem in that light in the ultraviolet region with a wavelength of 380 nm or shorter is not transmitted therethrough (herein, light with a wavelength of 380 nm to 220 nm is defined as ultraviolet light). On the other hand, in an acoustooptic device in which a TeO2 crystal is used, although the absorption edge wavelength is around 330 nm in the TeO2 crystal, there has been a problem in that it is not suitable for the use where high pulse peak power is used, as disclosed in Patent Document 1.
Furthermore, the acoustooptic device in which quartz glass, a quartz crystal, or a KDP crystal is used delivers poor acoustooptic performance. Hence, a high-frequency signal power source for driving the acoustooptic device is required, so that the acoustooptic device has to be water-cooled to suppress the generation of heat therein. Moreover, although an acoustooptic device that is usable with light in the ultraviolet region and highly resistant to laser damage can be realized by the use of a borate-based crystal, such an acoustooptic device may not exhibit sufficient acoustooptic performance depending on its use.
Furthermore, when an acoustooptic device is used with light having a short wavelength, it faces a problem in that dust may adhere to its light incident surface and light emitting surface. In the case of light having a still shorter wavelength, there has been a problem in that suspended substances present in the air are decomposed and the resultant decomposition products are adsorbed on the light incident surface and the light emitting surface.
Furthermore, when the acoustooptic device is produced in an environment with low humidity, it may adsorb a trace amount of dust etc. present in the air, which brings about a problem in that, when a portion where the dust is adsorbed is irradiated with a laser beam, the light transmittance may deteriorate in that portion or the dust may be burned onto the light incident surface or the light emitting surface of the acoustooptic device.
On the other hand, an optical imaging apparatus has been studied that uses light in the blue to ultraviolet region as an apparatus for direct drawing on an electronic circuit board or a light source of various types of printer, for example. In an optical imaging apparatus using a conventional acoustooptic device, its acoustooptic medium has to be provided with moisture resistance and water-cooling has to be performed to dissipate heat, so that the size of a driving circuit etc. is increased. Moreover, dust adheres to a light incident surface and a light emitting surface of the acoustooptic device, so that the optical imaging apparatus cannot be used continuously for a long time.
Patent Document 1: JP 2004-170940 A                Non-Patent Document 1: Proceeding IEEE Ultrasonic Sympo, Vol. 1998, pp. 1289-1292 (1998)        Non-Patent Document 2: Proceeding of the IEEE, Vol. 61, No 8 pp. 1073-1092 (1973)        